5 Abstract Malignant mesothelioma is a rare type of tumor that carries poor prognosis, being the life expectancy generally less than one year after diagnosis. Mesothelioma affects primarily the mesothelium that covers the lungs, but it can also affect the heart, abdomen and other organs. The most common approach to the cytoreduction of the primary tumor is surgery, but the majority of mesothelioma patients cannot tolerate it. Ablative procedures, such as the ones that use radiofrequency or ultrasound waves, become an attractive approach since they are much less invasive. In both cases, ablation is achieved by increasing the tissue temperature to over 60ºC, leading to cell death almost instantly. In the first case, however, the heat generated from a high frequency alternating current is used to ablate the damaged tissue, while in the second case the heat source is an ultrasound beam focused through a transducer completely external to the patient. The objective of this study was to determine and to compare the feasibility of transcutaneous and percutaneous mesothelioma debulking with Magnetic Resonance guided Focused Ultrasound Surgery (MRgFUS) and Radiofrequency Ablation (RFA), respectively, in a porcine model of mesothelioma. The tumor model was developed through the injection of a human mesothelioma cell line (MSTO-211H) in the right lower hemithorax of 13 Yorkshire pigs. Pigs were imaged using Magnetic Resonance Imaging (MRI) every 4 weeks post-inoculation. In T2- wighted MRI pulse sequences, signs of pleural effusion, presence of adhesions and an increase in diaphragm thickness were observed in several animals, then confirmed during post-treatment necropsy procedure. Five pigs were treated with RFA (guided by fluoroscopy imaging), while other four animals were treated with MRgFUS. The ablation areas obtained with both techniques were approximately the same, but MRgFUS has the advantage of being a non-invasive technique. Furthermore, treatment planning was more accurate since the position of the target areas were confirmed right v

6 prior to the treatment through MRI, which it was not possible with fluoroscopy imaging before RFA. With this study, we were able to create a successful mesothelioma tumor model in pigs and study its characteristics in-vivo using MRI and post-mortem during necropsy. Furthermore, it was possible to prove the feasibility to both techniques, as well as to investigate the best parameters to apply them in large animal. Keywords: Malignant pleural mesothelioma, tumor model, swine, Magnetic Resonance Imaging, Radiofrequency Ablation, Ultrasound, Magnetic Resonance-guided Focused Ultrasound Surgery. vi

11 Acknowledgements During my academic journey, several people have been by my side, showing their support and believing in my abilities. To all of them, I would like to write a few lines to show my gratitude. First of all, I m thankful for Professor Jaime Mata receiving me in the University of Virginia, for the second time. He greatly contributed for my education and showed me what doing research abroad is all about. I would also like to show my appreciation for professor Eduardo Ducla-Soares help, most of all for all the support and encouragement that he provided during the five years I ve been a student in the Faculty of Sciences. Professor Ducla-Soares was always the kind of teacher that believed in his students and, therefore he is an inspiration as an individual. I m also grateful for the friends I met in the University, Carolina, Rafael, Ana Sofia, Joana and Catarina, because they have been by my side in my (very few!) down moments, but even better because we celebrated together our achievements I leave a special word to Carolina, who shared a house with me across the Atlantic Ocean and made me grow so much as a person. Finally, I would like to dedicate this thesis for my parents and my brother for always believing in me. They are the people who know me better and they always made a great effort to provide me the economical and emotional structures that I needed to successfully achieve all my goals. xi

14 List of tables Table 1 Quantity of cell solution (10 6 cells/ml) injected in the liver and right pleural space, as well as the dose and ending time-point for cyclosporine administration Table 2 Evolution of pigs symptoms over time.33 Table 3 Degrees of disease severity Table 4 Macroscopic features observed in all pigs during necropsy. The severity of the disease was defined based on the number of features observed and their extension on pleural cavity, according to a reviewer. Stage one corresponds to the lowest severity degree and IV the highest...35 Table 5 Summary of the power applied in the pleural treatments and correspondent ablation size...45 Table 6 Summary of the power applied in the liver treatments and correspondent ablation size Table 7 Summary of the different parameters of the MRgFUS treatments in the pleura and correspondent ablation sizes (Values of power, duration and energy parameters correspond to each focal spot) 55 Table 8 Summary of the different parameters of the MRgFUS treatments in the liver and correspondent ablation sizes (Values of power, duration and energy parameters correspond to each focal spot)...60 xiv

15 List of Figures Figure 1 Asbestos-related deaths in United States (2002) comparatively with other conditions. The number of deaths is superior to conditions such as skin cancer, asthma, hepatitis and Hodgkins disease... 3 Figure 2 - Staging on Malignant Pleural Mesothelioma, by characteristics of tumor, lymph nodes and metastases..5 Figure 3 Contrast enhanced coronal T1-weighted image and axial LAVA images of a patient diagnosed with pleural mesothelioma, revealing circumferential pleural nodules, without diaphragmatic (white arrow) or chest wall invasion....7 Figure 4 A: RFA circuit. Electrode acts like the cathode and the grounding pads as the anode. The procedure is very dependent on tissues electric and thermal conductivity since the patients is part of the circuit; B: The RFA electrode produces an alternating electromagnetic field, resulting in the adjacent molecules motion, that are act as a source of heat for the RFA procedure 10 Figure 5 Comparison between the ablation results with slower and faster temperature rise. The final ablation area of the top row is larger than the one in the bottom row, since the faster temperature increase resulted in desiccated tissue around electrode tip before the maximum ablation area has been achieved. Further deposition of energy in the adjacent tissue is hard to obtain because resistance become too high.10 Figure 6 Different types if RFA probes. A: Christmas tree configuration by RITA Medical Systems; B: Umbrella shaped array from Radiotherapeutics, similar to the one used in this project; C: Single cooled-tip needle from Radionics...11 Figure 7 Color simulation of a cross section of an RF ablation local temperature effect: on the left, an image without any blood vessel; on the right, an image simulation temperature disturbance due to the presence of a blood vessel (white) Figure 8 Results of several studies about Percutaneous Radiofrequency Ablation of Hepatocellular Carcinoma...13 Figure 9 CT scans from a 46-year old patient with metastatic colon cancer. A: Before the treatment a 3-cm metastasis is observed in the posterior right lobe of the liver. B: Immediately after the ablation a thermal injury is observed in the treated area and there is no evidence of residual tumor. C: Follow-up 3 months after ablation shows recurrent tumor in the margins of the ablated area. D: Immediately after reablation, it is possible to observe an enlarged thermal injury with no evidence of recurrent tumor 13 xv

16 Figure 10 A: HIFU-induced biological effects by hyperthermia. US waves are focused into a small spot, where acoustic pressure increases and consequently rising tissue temperature. B: Properties of a geometrically focused transducer. C: Normalized acoustic pressure in the direction of ultrasound propagation for a 1.5MHz transducer with a radius of curvature of 8 cm and diameter of 10 cm 16 Figure 11 Different configurations for the use of phases-arrays transducers to produce multiple focal spots, steer the focal spot to different locations and correct aberrations...17 Figure 12 Different transducers for focusing ultrasound. A: Spherically-curved transducer; B: Flat transducer with interchangeable lens; C and D: phased-array transducers 17 Figure 13 Temperature images acquired during sonication to evaluate targeting. A and B: Images acquired before correcting focal coordinate; C and D: images acquired after correction. A and B: images acquired with temperature imaging perpendicular to the ultrasound beam direction. C and D: images acquired with temperature mapping along the beam direction 19 Figure 14 MRgFUS of hepatocellular carcinoma. Left: Post-treatment image in coronal plane showing entire thermal dose. Right: contrast-enhanced MRI showing focal nonperfusion...21 Figure 15 Setup environment for cell inoculation. Fluoroscopy allows to clearly distinguishing the liver, the heart and the ribs, therefore guiding the procedure. Inoculations were performed in the right side in the intercostal spaces..24 Figure 16 Time line of the procedures. Pigs were treated at different time points.25 Figure Radio Frequency Generator 26 Figure 18- Umbrella needle used for the RFA treatments. Left: umbrella needle from Boston Scientific. Right: 2.0 cm needle used for the project...26 Figure 19 Software for MRgFUS treatment planning by Insightec. The region colored green corresponds to the planned ablation area (pig #8). The green square represents the position of the transducer. Sonication spots were defined within this region...27 Figure 20 Left: The blue circles above the diaphragm represent two the focal spots planned to treat the lesion, both localized inside the green square. Right: Saggital view of the image used for treatment planning. The rectangles represent the depth of the focal spots. The blue region corresponds to the beam path across the adjacent tissue 28 Figure 21 Main mesothelioma findings in MRI. Left: True FISP transversal image of pig #8 showing pleural thickening (blue arrow); Middle: True FISP coronal image of pig #8 showing pleural fluid (yellow arrow) and the increased thickness of diaphragm (red arrow); Right: True FISP coronal image of pig #8 showing adhesions (orange arrow)..30 xvi

17 Figure 22 Macroscopic findings during necropsy: adhesions (blue arrow), increased diaphragm thickness (red arrow), abnormal color in the lungs (purple arrow) and increased pericardium thickness (green arrow)..34 Figure 23 A: MR image (True FISP) used for treatment planning in the right (white arrow) and the left (blue arrow) sides. B: Needle placement in the right side (Fluoroscopy image); C: Postablation MRI (True FISP), showing the ablated area in the right side (green arrow); D: Necropsy image showing the ablated area in the right side (purple arrow). E: Needle placement in the left side (Fluoroscopy image); F: Post-ablation MRI (True FISP), showing the ablated area in the left side (red arrow); G: Necropsy image showing the ablated area in the left side (yellow arrow) 38 Figure 24 A: MR image (True FISP) used for treatment planning in the right (white arrow) and the left (blue arrow) sides. B: Needle placement in the right side (Fluoroscopy image); C: Postablation MRI (True FISP), showing the ablated area in the right side (green arrow); D: Necropsy image showing the ablated area in the right side (purple arrow). E: Needle placement in the left side (Fluoroscopy image); F: Post-ablation MRI (True FISP), showing the ablated area in the left side (red arrow); G: Necropsy image showing the ablated area in the left side (yellow arrow) 40 Figure 25 A: MR image (True FISP) used for treatment planning in the right (white arrow) and the left (blue arrow) sides. B: Needle placement in the right side (Fluoroscopy image); C: Post-ablation MRI (True FISP), showing the ablated area in the right side (green arrow); D: Necropsy image showing the ablated area in the right side (purple arrow). E: Needle placement in the left side (Fluoroscopy image); F: Post-ablation MRI (True FISP), showing the ablated area in the left side (red arrow); G: Necropsy image showing the ablated area in the left side (yellow arrow) 42 Figure 26 A: MR image (True FISP) used for treatment planning in the right (white arrow) and the left (blue arrow) sides. B: Needle placement in the right side (Fluoroscopy image); C: Postablation MRI (True FISP), showing the ablated area in the right side (green arrow); D: Necropsy image showing the ablated area in the right side (purple arrow). E: Needle placement in the left side (Fluoroscopy image); F: Post-ablation MRI (TrueFISP), showing the ablated area in the left side (red arrow); G: Necropsy image showing the ablated area in the left side (yellow arrow).43 Figure 27 A: MR Needle placement in the right side (Fluoroscopy image); B: Post-ablation MRI (True FISP), showing the ablated area in the right side (green arrow); D: Necropsy image showing the ablated area in the right side(purple arrow). E: Needle placement in the left side in the 1st and 2nd positioning of the probe (Fluoroscopy image); F: Post-ablation MRI (True FISP), showing the ablated areas in the left side (red and blue arrows); G: Necropsy image showing the ablated areas in the left side (yellow and white arrow) 45 xvii

18 Figure 28 A: MR image (TWIST 4D) used for treatment planning in the liver (white arrow) B: Needle placement in the liver (Fluoroscopy image); C: Post-ablation MRI (True FISP), showing the ablated area in the liver (green arrow); D: Necropsy image showing the ablated area in the liver (purple arrow) Figure 29 A: MR image (VIBE) used for treatment planning in the liver (white arrow) B: Needle placement in the liver (Fluoroscopy image); C: Post-ablation MRI (True FISP), showing the ablated area in the liver (green arrow); D: Necropsy image showing the ablated area in the liver (purple arrow).48 Figure 30 A: MR image (TWIST 4D) used for treatment planning in the liver (white arrow) B: Needle placement in the liver (Fluoroscopy image); C: Post-ablation MRI (True FISP), showing a very small ablated area near the gallbladder in (green arrow); D: Necropsy image showing the ablated area in the liver (purple arrow). Probably, during the treatment probe was moved into the gallbladder..48 Figure 31 A: MR image (TWIST 4D) used for treatment planning in the liver (white arrow); B and D: Needle placement in both target areas (Fluoroscopy image); C and E: Post-ablation MRI (True FISP), showing the resultant two ablated areas (green and yellow arrows); F: Necropsy image showing two ablated area in the liver (purple and red arrows) 49 Figure 32 A: MR image (VIBE, transversal) used for treatment planning in the liver (white arrow) B: Needle placement in the liver (Fluoroscopy image); C: Post-ablation MRI (True FISP), showing a very large ablated area in the liver (green arrow); D: Necropsy image showing a large ablated area in the liver (purple arrow)...51 Figure 33 A: MR image (FIESTA) used for treatment planning in the pleura (white arrow) B: Post-ablation MRI (subtraction of LAVA images before and after ablation), showing a very small ablated area near the heart (green arrow); C: Necropsy image showing the ablated area in the pleura (purple arrow).53 Figure 34 A: MR image (FIESTA) used for treatment planning in the pleura (white arrow) B: Post-ablation MRI (subtraction of LAVA images before and after ablation), showing a very small ablated area near the heart (green arrow); C: Necropsy image showing the ablated area in the pleura (purple arrow).53 Figure 35 A: MR image (Localizer) used for treatment planning in the pleura (white arrow) B: Post-ablation MRI (subtraction of LAVA images before and after ablation), showing a very small ablated area near the diaphragm (green arrow); C: Necropsy image showing the ablated area in the pleura (purple arrow)..54 xviii

19 Figure 36 A: MR image (FIESTA) used for treatment planning in the pleura (white arrow) B: Post-ablation MRI (subtraction of LAVA images before and after ablation), showing a very small ablated area near the diaphragm (green arrow); C: Necropsy image showing the ablated area in the pleura, in the diaphragm (purple arrow) and in the surface of the lung (yellow arrow) 55 Figure 37 Burns were observed in 3 of the 4 pigs treated with MRgFUS. Left: pig # 10 revealed a burn in the muscle between skin and sternum; Middle: skin burn of pig #12; Left: skin burn of pig # Figure 38 A: MR image (VIBE) used for treatment planning in the liver (white arrow) B: Postablation MRI (subtraction of LAVA images before and after ablation), showing a very small ablated area far below the target area (green arrow); C: Necropsy image showing the ablated area in the liver (purple arrow).57 Figure 39 A: MR image (VIBE) used for treatment planning in the liver (white arrow) B: Postablation MRI (subtraction of LAVA images before and after ablation), showing a very small ablated area near the diaphragm (green arrow); during necropsy, no ablated area was observed..58 Figure 40 A: MR image (VIBE) used for treatment planning in the liver (white arrow) B: Postablation MRI (subtraction of LAVA images before and after ablation), showing a large ablated area correspondent to the target area (green arrow); C: Necropsy image showing the ablated area in the liver (purple arrow).59 Figure 41 A: MR image (VIBE) used for treatment planning in the liver (white arrow) B: Postablation MRI (subtraction of LAVA images before and after ablation), showing a large ablated area correspondent to the target area (green arrow); C: Necropsy image showing the ablated area in the liver (purple arrow).60 xix

21 Introduction Malignant Pleural Mesothelioma (MPM) is an aggressive, although relatively rare, tumor, arising from the surface serosal cells of the pleural cavity. Epidemiological studies reveal that the exposure to asbestos fibers is the main cause of mesothelioma. The patients affected by this condition carry a poor prognosis (generally they survive less than one year) so it is important the development of imagiology techniques. So far, the preferred techniques in the assessment of the disease are Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). MRI, besides avoiding the use of radiation in the already debilitated patients, also permits to obtain images with better contrast. Although the use of asbestos has been forbidden in most developed countries since the 70 s, this type of cancer has a long latency period that ranges from 20 to 50 years, which means that for most countries the peak in mesothelioma cases is predicted to take place in the period between 2010 and Therefore, developed countries have a great social and economic interest in the research of an effective therapy in the next years. Recently, multimodality therapy has been used to treat mesothelioma patients. Its main goal is the cytoreduction of the primary tumor by surgery, radiation or ablative therapy and hence, the volume of tumor debulking must be maximized. The primary option for mesothelioma cytoreduction is surgery. However, 90% of mesothelioma patients cannot tolerate it, since by the time they are diagnosed their general health condition is very poor. In this sense, ablative procedures, such as the ones that use radiofrequency or ultrasound waves, become an attractive approach. While Radiofrequency Ablation has been widely used in clinical practice for the treatment of solid tumors (particularly hepatocellular carcinoma in the liver) as well as other heart conditions, Focused Ultrasound only in the last decade got the approval from Food and Drug Administration for the treatment of uterine fibroids. This technique, however, has showed promising results and, 1

22 therefore, a wide range of applications are being investigated and it is one of the most groundbreaking non-invasive cancer therapies. The first aim of this project is to develop and to characterize a successful mesothelioma tumor model in pigs (12 animals). Animals will be divided in two groups (each group with six animals) and one pig from each group will be used as a control animal to characterize the tumor model. The second aim of this project is to treat one group of animals with RFA and the other group with MRgFUS, and therefore to investigate the feasibility of percutaneous RFA and transcutaneous MRgFUS in a swine mesothelioma tumor model. The first chapter of this dissertation will be a theoretical background of the aforementioned disease and the ablative procedures. The second and third chapters will describe the methods used to achieve the goals proposed and the results obtained with the experiment, respectively. In the fourth chapter, the successful points and pitfalls of the study, as well as the comparison between both therapies, will be discussed. Finally, the future directions of MRgFUS will be briefly approached in the section dedicated to the final conclusions. 2

23 CHAPTER I Malignant Pleural Mesothelioma Malignant Pleural Mesothelioma (MPM) is an aggressive type of cancer that develops from mesothelial cells lining in the pleura. Epidemiological studies reveal that the exposure to asbestos fibers, from industrial and environmental sources, is the main cause of mesothelioma [1, 2]. Although it is believed that in the United States the peak of mesothelioma occurred in 2004, this condition is still a problem worldwide, with an increasing number of diagnosed cases in Europe every year [1]. Figure 1 represents the number of deaths in the United States in 2002, due to some of the diseases that affect a larger number of people in this country and asbestos-related deaths are higher than other respiratory diseases, such as asthma and tuberculosis. Figure 1 Asbestos-related deaths in United States (2002) comparatively with other conditions. The number of deaths is superior to conditions such as skin cancer, asthma, hepatitis and Hodgkins disease [3]. 3

24 Asbestos utilization was forbidden in most developed countries in the70 s. Malignant pleural mesothelioma, however, is associated with a long period of latency between the exposure and the expression of the disease, ranging from 20 to 50 years, since the mineral fibers get lodge in the pleura and the organism cannot eliminate them easily, which permits them to be embedded in the pleural space for decades, continuously damaging the tissue [4]. This means that for most European and Asiatic countries the peak is predicted between 2010 and 2020 and it is expected more than 250,000 deaths in Western Europe and more than 100,000 in Japan. Patients have a dismal prognosis and the average survival time ranges between 4 and 12 months, regardless of stage [1]. MPM occurs more frequently in men and risk increases with age [1,2]. MPM can be histologically classified in three subtypes: epithelial, the most common type with 50% to 60% of cases, sarcomatoid, comprising 10% of cases and biphasic, the combination of the other two types. Epithelial mesotheliomas have a better prognosis than the two other forms. On the other hand, sarcomatoid mesotheliomas are very resistant to therapy [1]. Besides the long latency period, pleural mesothelioma has a difficult diagnosis and it is very challenging to understand the early events in the malignant mesothelioma development [5]. A study by Hiroshima et al. [6] evaluated the early histopathological characteristics of mesothelioma and found out in eight early stage patients (six with epithelioid and two with biphasic mesothelioma) that macroscopically there was no visible tumor, but both visceral and parietal pleura were thickened and several adhesions were observed between them. Therefore, in early stages, it is not always possible to distinguish tumor masses, but adhesions and pleural effusion. In the advanced stages, however, nodules become larger and more confluent. Approximately 30% of pleural mesothelioma cases directly invade the parietal pericardium [6]. Metastatic disease is often associated with pleural mesothelioma. The most common site is the lymph nodes, followed by the visceral pleural surface of the contralateral lung. Mesothelioma may also invade the diaphragm and even progressively encase organs in the abdominal cavity, such as the liver, brain and bone [1, 2]. 4

25 Until now, there are no screening tests to detect the early stages of mesothelioma, but there are three staging systems widely used to evaluate this condition. The International Mesothelioma Interest Group (IMIG) defined a staging system based on the characteristics of the tumor, lymph node involvement and metastatic disease, represented on figure 2 [7]. MPM is very difficult to treat, partly because there are few clinical trials, since the number of patients is relatively small compared to other cancer types, the different histological characteristics between patients and also a mismatch between the radiographic and surgical information [1, 5]. However, since the increase of diagnosed cases in the last years, several studies investigated the different approaches in the treatment of both resectable and unresectable disease [1,2]. In the first case, surgery is the primary option, generally combined with adjuvant radiotherapy or chemotherapy. On the other hand, in case of unresectable Figure 2 - Staging on Malignant Pleural Mesothelioma, by characteristics of tumor, lymph nodes and metastases [7]. disease, chemotherapy has been commonly used and more recently biological therapy. 5

26 For MPM patients, two types of surgery are performed, pluerectomy/decortication (P/D) and extrapleural pneumonectomy (EPP), both very aggressive procedures. P/D is an open thoracotomy for removal of parietal and visceral pleura as well as the mesothelium covering the pericardium and diaphragm. EPP consists of removal of the affected tissue in the hemithorax, including visceral and parietal pleura, diseased lung, mediastinal lymph nodes, pericardium and diaphragm [1, 2, 5]. This procedure is associated with higher morbidity rates. One of the problems with surgery is both local and distant recurrence of the tumor, because neither of the procedures has the ability of completely eliminating the residual microscopic disease [1]. Therefore the use of adjuvant therapies is necessary to complete the treatment. Radiotherapy is used either in the surgical incision sites to prevent tumor seeding or in the entire hemithorax to avoid the recurrence of the tumor in the thoracic cavity and the spread of tumor cells to distant organ. This is the only adjuvant therapy that prevents local recurrence of the disease. Chemotherapy may also be used as an adjuvant or neoadjuvant therapy for resectable disease, concurrent with radiotherapy, permitting an overall survival variable between 16.6 and 25.5 months [1]. Chemotherapy is, nevertheless, the primary choice for MPM treatment when the disease is unresectable, although MPM is more resistant to this kind of treatment than other tumors. Cisplatin has been the most common agent used [1, 2]. However, several studies showed that the combination of cisplatin with other agents, such as pemetrexed, increased the overall survival rate from 9.5 months using only cisplatin to 12.1 months, as well as a general increase of patients life quality in the first weeks of treatment. Until this point, pemetrexed has been used as front-line chemotherapy and no salvage regimen has been approved yet to MPM. Jassem et al. however conducted a phase III clinical trial to prove that a salvage setting in pemetrexed administration improves tumor response, although they were not able to show an increase in the overall survival for patients that did not have a positive response in a front-line regimen of chemotherapy [1]. Several imagiology techniques are available for the diagnosis of MPM. Computed Tomography (CT) has been used as the primary imaging modality for the diagnosis, staging and 6

27 monitoring of mesothelioma treatment. Magnetic Resonance Imaging (MRI), however, provides additional information to CT because of the excellent contrast resolution, advantageous in the differentiation of malignant from benign disease and metastatic involvement of chest wall and diaphragm [8]. Perfusion MRI has been widely used to assess tumor vascularity and vascular permeability. To detect tumor invasions of the adjacent structures it is common to use contrast-enhanced T1 fatsuppressed sequences. High signal intensity comparatively to the adjacent musculature suggests malignant disease. High intensity is visible in T1-weighted (T1-w) and T2-weighted (T2-w) sequences (figure 3). Furthermore, it is possible to distinguish pleural effusion, frequently observed as focal areas of very high signal intensity on T2- w images. The most promising MR approach, however, is to use perfusion MRI, with the injection of a contrast agent. Generally, in clinical practice, gadolinium (Gd) is the chosen contrast agent [8, 9]. Figure 3 Contrast enhanced coronal T1-weighted image and axial LAVA images of a patient diagnosed with pleural mesothelioma, revealing circumferential pleural nodules, without diaphragmatic (white arrow) or chest wall invasion [9]. 7

28 To perform MR imaging in mesothelioma patients one has to take in consideration that the protocol needs to be done in a reasonable time frame (in 15 to 30 minutes). The most common approach to MR image mesothelioma protocols is to use fast sequences for single or multiple breath-hold imaging, permitting a reasonably high spatial resolution imaging and short echo time (TE). The main goal is to obtain as much signal as possible before the signal decays [8,9]. 8

29 CHAPTER II Ablative Therapies 2.1 Radiofrequency Ablation Radiofrequency ablation is a minimally invasive tumor treatment, mostly used in clinical practice to treat small renal masses [10]. During the procedure, the ablation needle is placed directly in the target tissue and one or more electrodes are deployed from the tip of the needle [10, 11]. These electrodes have small thermocouples responsible for temperature monitoring. The needle is connected to a generator capable of producing an alternating current that flows through the electrodes into the tissue, therefore exposing tissues to an electromagnetic (EM) field (figure 4) [10]. The dipole molecules in the tissue, mostly water, adjacent to the RF electrode, vibrate rapidly in the direction of the alternating current. Molecules further away in the other hand move by the molecules vibrating near them, which consequently causes a frictional energy that deposits heat and increases the temperature in the tissue. When 60ºC are achieved, protein denaturation occurs and the coagulation necrosis starts, resulting in a quasi-instantly cell death. Tissue ablation due to thermal effects occurs because the cells in soft tissue lose their ability to conduct electric current, which causes an increase in tissue impedance [10, 11]. The majority of the heat deposits in the tissue surrounding the probe. An exponential decrease in the temperature happens moving away from the probe due to the poor conductivity of tissues. Besides the small active electrode placed in the target zone, radiofrequency ablation procedure makes use of a large dispersive electrode that closes the circuit and it is used so that the current could pass freely in the patient (figure 4), without a significant increase in the heat except in the tip of the RFA probe due to their larger surface area comparatively with the tip [10]. 9

30 A B Figure 4 A: RFA circuit. Electrode acts like the cathode and the grounding pads as the anode. The procedure is very dependent on tissues electric and thermal conductivity since the patients is part of the circuit; B: The RFA electrode produces an alternating electromagnetic field, resulting in the adjacent molecules motion, that are act as a source of heat for the RFA procedure [10]. The radiofrequency generator power and depth of the lesions are limited for thrombus formation that happens when the electrode-tissue temperature rises above 80ºC. At 100ºC death cells occurs as evaporation and microbubbles containing nitrogen are produced, resulting in desiccated (or charred) tissue [10, 11]. Adherent tissue is a common finding when removing the probe due to the temperature rising above the aforementioned threshold, which results in boiling of the plasma and adherence of denatured proteins to the probe tip (figure 5). The desiccated tissue acts as an insulator that prevents the current to move to the adjacent organs, therefore compromising the procedure. Figure 5 Comparison between the ablation results with slower and faster temperature rise. The final ablation area of the top row is larger than the one in the bottom row, since the faster temperature increase resulted in desiccated tissue around electrode tip before the maximum ablation area has been achieved. Further deposition of energy in the adjacent tissue is hard to obtain because resistance become too high [10]. 10

31 To avoid the problem of tissue desiccation and scarring, several models of needle designs and generator programs were developed. Some electrodes have a Christmas tree configuration being possible to extend and retract the electrodes of the adjacent tissue (figure 6A). Other type of needles has retractable electrodes, that when deployed assume an umbrella form (figure 6B). Other types include the use a hollow needle with an exposed and closed tip of variable length which contains a thermocouple (figure 6C). Most electrodes used in clinical practice use a monopolar configuration. More recently, however, bipolar systems were developed in which two or more electrode are placed into or around the tumor, without being required the use of grounding pads, since the current flows from one electrode to the other [12, 13]. Several RF generator systems are also available to support the design of the different electrodes, using either temperature or impedance to maximize the diameter of the ablated area and each one with a specific treatment algorithm. Figure 6 Different types if RFA probes. A: Christmas tree configuration by RITA Medical Systems; B: Umbrella shaped array from Radiotherapeutics, similar to the one used in this project; C: Single cooled-tip needle from Radionics [12]. A common problem when using RFA thermal ablation is the presence of large blood vessels (generally 3 mm or larger) due to the «heat sink» effect resultant from the heat lost, consequence of the cooling effects of blood flow. This undesirable effect limits the ablation area; 11

32 hence some residual tumor margins may be untreated, increasing the odds of local tumor recurrence [10, 14]. The preoperative evaluation is generally made based in CT or MR imaging used to study the number and size of tumor masses and their position relatively to sensible structures such as blood vessels (figure 7). The most common approach for the RFA procedure is a percutaneous treatment because is the least invasive, with an associated low morbidity rate and it can be performed on an outpatient basis since only conscious sedation is required. However, both laparoscopy and laparotomy RFA have been used to treat liver tumors. For the needle placement, in clinical practice, US, CT and MRI are used to guide the treatment, although other techniques may be used. US is the preferred method, but has limited ability to assess the effectiveness of the treatment providing only a rough estimation of the ablation spot size. Furthermore, this technique does not permit to obtain images with defined tumor margins and consequently they are not removed which does not prevent local recurrence of the tumor. Almost all patients feel transient side effects, nausea and pain, during and after the procedure. Some of the patients (25%) also have some late flu-like symptoms, such as fever and general malaise, 3 to 5 days after the ablation. Figure 7 Color simulation of a cross section of an RF ablation local temperature effect: on the left, an image without any blood vessel; on the right, an image simulation temperature disturbance due to the presence of a blood vessel (white). The most common application for RFA is the treatment of the liver, in both hepatocellular carcinoma and metastatic liver tumors. When ablating the tumor, besides the blood vessels, the 12

33 physician may also plane the treatment avoiding bile ducts, gallbladder, diaphragm and bowel, because patients with masses near these structures feel more pain during and after the ablation. The main concern when ablating near the blood vessels is that the blood flow in the vessels cools the tissue reducing the heating and therefore decreasing the effectiveness of the treatment. Patients are considered candidates if they have less than 5 masses, each less than 5 cm in diameter and no evidence of extrahepatic tumor. Figure 8 shows the results of RFA procedures in hepatocellular carcinoma patients. Rossi et al. [12] showed that it was possible to achieve complete necrosis in almost 90% of the patients with hepatocellular carcinoma in 6 months when treating tumors with a diameter smaller than 3 cm. Livraghi et al. [12]on the other hand conclude that for tumor larger than 3 cm the complete necrosis success decreased to 71% (3.1-5 cm). Figure 8 Results of several studies about Percutaneous Radiofrequency Ablation of Hepatocellular Carcinoma [12]. Figure 9 CT scans from a 46-year old patient with metastatic colon cancer. A: Before the treatment a 3-cm metastasis is observed in the posterior right lobe of the liver. B: Immediately after the ablation a thermal injury is observed in the treated area and there is no evidence of residual tumor. C: Follow-up 3 months after ablation shows recurrent tumor in the margins of the ablated area. D: Immediately after re-ablation, it is possible to observe an enlarged thermal injury with no evidence of recurrent tumor [12]. 13

34 Radiofrequency ablation has also been used for lung cancer treatment. Simon et al. [15] in 2007 published a study about long-term survival, local tumor progression and complications rates in 153 patients submitted to CT-guided RFA in the lung between 1998 and The overall conclusions were that the treatment has promising results in long-term survival rates (1-, 2-, 3-, 4-, and 5-year survival rates, respectively, were 78%, 57%, 36%, 27%, and 27%) and local tumor progression outcomes (1-, 2-, 3-, 4-, and 5-year local tumor progression free rates, respectively, were 83%, 64%, 57%, 47%, and 47% for tumors 3 cm or smaller and 45%, 25%, 25%, 25%, and 25% for tumors larger than 3 cm). Nevertheless, there was a significant difference between the survival curves of patients with large (> 3cm) and small tumors, a common finding in studies with RFA. Research on RFA for kidney, breast and bone tumors has also been widely developed. Besides tumor ablation, RFA has been applied in cardiac arrhythmia and pain management. RFA was introduced in clinical use for cardiac arrhythmia to replace the direct-current shocks, therefore avoiding the stimulation of cardiac and skeletal muscle, minimum discomfort and the discrete resultant lesions from ablation that combined with the short treatment time reduce post procedural complications [16]. Therefore it presents the advantages of symptoms relief, improvement in the quality of life, discards the need for lifelong antiarrhythmic-drug therapy and allows long-term cost savings. 2.2 High Intensity Focused Ultrasound Therapy High Intensity Focused Ultrasound (HIFU) therapy is an emergent, non-invasive treatment with great potential for tumor ablation, hemostasis, thrombolysis and targeted drug/gene delivery [17, 18]. This therapy relies on the effects of Ultrasound (US) waves when focused on specific target tissue of the body, resulting in an increase of tissues temperature that play an important role in tumor ablation and hemostasis, or other non-thermal effects, significantly important in 14

35 thrombolysis or targeted drug/gene-delivery through cavitation mechanisms. For these last applications low intensities US are used [17]. Ultrasound has a frequency higher than human ear can detect (superior to 20,000 Hz). By definition, sound is a disturbance of mechanical energy travelling to a medium, which implies that a medium has to be present for sound propagation. As the ultrasound wave propagates molecules within the medium oscillate around their rest position in the direction of the wave, forming compressions and rarefactions that propagate the wave. The ultrasound energy attenuates exponentially as the wave travels through the tissue [17]. An important parameter when characterizing ultrasound wave propagation is the acoustic intensity that it is defined as the rate of energy flow through a unit area, normal to the direction of the propagation. US have been mostly used in a diagnosis sense, showing minimal effects in the biological tissue. However, by being able to maximize the energy accumulation in a specified target point it is possible to induce significant changes in the definite biological tissue while sparing the adjacent organs as well as the skin [17]. As aforementioned, thermal effect and acoustic cavitation are the mechanisms more significant in US and, therefore the resulting reactions in tissue by these effects have been widely investigated and are now relatively well-understood. The first one results from the absorption of US energy by the tissue, since the waves cause vibration and rotation of the molecules present in the tissue and consequently frictional heat is generated. The US-induces changes by hyperthermia are dependent on the temperature (T) reached and the duration of beam contact in the tissue. Hence, the result may be an increase in tissues susceptibility for chemotherapy or radiotherapy (T>43ºC during 1 hour) or protein denaturation, also referred as coagulative necrosis (T=56ºC during 1 second) [17]. The increase in the temperature is linearly-proportional to sonic intensity, as demonstrated by equation 1 [17]: (1) 15

36 being T = temperature (ºC), t = time (sec), α = absorption coefficient (~ 0.03 Np/cm in tissue-like medium at 1MHz), I = sonic intensity, ρ = density (~ 1g.cm -3 in tissue-like medium) and C p = specific heat (~ 4.2 J/gºC in tissue-like medium). The ultrasound beam is focused to obtain very small focal spots, which allows a very precise treatment (figure 10). For example, using a 1.5 MHz transducer, it is possible obtain a focal spot with 1 mm of diameter. The length of the focus is generally 5 to 20 times larger than the diameter []18. Focusing the beam allows to concentrate US energy deep in the tissue, sparing the adjacent organs and avoiding the ultrasound attenuation across the area within the sonication path. To improve beam focusing, transducer arrays have been developed containing signals with different phase to obtain a common focal spot. These systems permit electrical focusing then allowing multiple focal spots simultaneously which contributes to decrease treatment time (figures 11 and 12) [18]. 43ºC 56ºC Figure 10 A: HIFU-induced biological effects by hyperthermia. US waves are focused into a small spot, where acoustic pressure increases and consequently rising tissue temperature. B: Properties of a geometrically focused transducer. C: Normalized acoustic pressure in the direction of ultrasound propagation for a 1.5MHz transducer with a radius of curvature of 8 cm and diameter of 10 cm [18]. 16

37 Figure 11 Different configurations for the use of phases-arrays transducers to produce multiple focal spots, steer the focal spot to different locations and correct aberrations [18]. Figure 12 Different transducers for focusing ultrasound. A: Spherically-curved transducer; B: Flat transducer with interchangeable lens; C and D: phased-array transducers [17]. Interfaces between different types of tissue are a problem in HIFU. Ultrasound is transmitted between surfaces of soft tissue with a small amount of wave reflected back. Between soft tissue-bone the amount increases to one third of the incident energy [19]. Furthermore, the amplitude attenuation coefficient of US is 10 to 20 times higher in bone than in soft tissue, which means that the beam is rapidly absorbed within the bone [17]. The main problem however occurs in the interface between gas and tissue, because gas has lower density than water or soft tissue and therefore almost all energy is reflected back. This is the main reason why (so far) lungs have not been considered an option for HIFU treatment [20]. Acoustic cavitation results in the formation of gas or vapor filled bubbles. This occurs when a US wave with an intensity superior to a specified threshold interacts with biological tissue 17

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